1. Field of the Invention
This invention relates to a heater unit which heats an object to be heated such as a semiconductor substrate for example.
2. Description of the Related Art
A heater unit is proposed which heats a wafer which is a semiconductor substrate, for example, and an example of this is revealed in a patent document, Japanese Laid-Open patent Publication No. 2005-216759.
In this type of heater unit, a heater plate is supported by a shaft part and a resistance heating element is routed through the inner section of the heater plate passing through the shaft part. It is a mechanism whereby when a current is passed along the resistance heating element, the resistance heating element generates heat and by this heat a wafer mounted on the heater plate is heated.
However, when repeatedly switching between a heated state and a non heated state of the resistance heating element by controlling the current passed through the resistance heating element, more time is required to heat the area which confronts the hollow area (the opening formed in the direction of the shaft's axis) which is formed in the shaft within the heater plate until a desired heating temperature than to heat other areas. As a result, the temperature of the wafer which is mounted on the plate differs in horizontal direction.
In consideration of the above stated fact, this invention aims to obtain a heater unit which can improve uniformity in the temperature of an object concerned at the time of heating the object to be heated.